My LQS owner told me when I tea dye not to use decaf tea. She said it didn't do as well. And she liked tea leaves better than bags. Otherwise she said to do it just like with any dye, put fabric in the mix and leave it until it is a shade darker than the color you want (because rinsing will take some color away). Rinse in clear water, and set with heat. I did what she said and got a great color.
Oh, I got some of those towels from Sam's. They don't have the same weave as the old feed sacks. They don't take color as easily when you dye them, either. Be sure you dampen them before you start.